This type of the heavy duty pneumatic tire is required to have a higher traction performance and a considerably severer durability as compared with tires for truck and bus and tires for light truck, which are run on expressways. For this end, the conventional heavy duty pneumatic tire generally has such a construction of a tread pattern that lug grooves aiming at the improvement of the traction performance are arranged in each side region of the tread and a rib contributing to the improvement of the durability to heat buildup is arranged in a central region of the tread.
In such a construction, however, there is known that as the width of the rib in the central region of the tread is too wide, the deformation of the rib itself can be suppressed to decrease the heat buildup quantity of the rib, but the quantity of frictional energy on a road surface converted to rib deformation energy and others is decreased to lower the wearing life of the rib and hence the tread.
Therefore, there is recently proposed a technique that a block row is arranged in the central region of the tread for the purpose of simultaneously establishing the improvement of the wearing life in the central region and the decrease of the heat buildup quantity and blocks in the block row are defined by grooves having a narrower width as compared with the conventional technique.
According to this technique, the heat buildup at the central region of the tread can be suppressed to a low value by contacting groove walls of the narrow-width groove and hence the mutually adjoining blocks with each other in a ground contact zone of the treading face on the road surface to control the deformation quantity as a whole of the block row, and also the heat dissipation effect at the central region of the tread can be enhanced based on fact that the groove walls of the narrow-width groove are separated away from each other at a zone other than the ground contact zone, and as a result, the thermal deterioration of the central region of the tread can be advantageously prevented.
In addition, the lowering of the friction life in the central region of the tread can be effectively prevented by converting a greater part of friction energy of the treading face on the road surface into a block deformation energy and the like.
However, the thickness of the tread in the heavy duty tire for the off-the-road is very thick, so that it is very difficult to adequately form the narrow-width grooves at the required depth in the tread. For example, the formation of the narrow-width grooves and the like in the treading face of the green tire and the vulcanization of the tread portion are carried out by a plurality of segment-shaped sector molds arranged at predetermined pitches in the circumferential direction of the tread and displacing extendedly and retractively in the radial direction of the tire, there is a problem that when a projection for the formation of the groove arranged in the inner surface of the each sector mold is inserted into the tread portion of the green tire in the radial direction of the tire, the bending, breakage or the like is easily caused in the projection for the formation of the narrow-width groove having a thin thickness and a long projecting length, and even if the insertion of the projection into the green tire is properly conducted, after the completion of the vulcanization to the tread portion, when the each sector mold is displaced extendedly and separated off from the vulcanized tire, the projection for the formation of the narrow-width groove inserted into the treading face in the radial direction is subjected to a large drawing resistance from the vulcanized and cured tire to easily cause the deformation or the like and also the land portion of the tread is easily broken by the projection.
The invention is to solve the above problems of the conventional technique and to provide a tire vulcanizing mold capable of smoothly and surely forming grooves having a narrow width and a deep depth in the treading face without causing the damage, breakage and the like of the projection for the formation of the groove in the sector mold and the land portion of the tread when a tread portion of a large-size heavy duty tire is vulcanized by using the sector molds, and a pneumatic tire vulcanized and shaped therethrough.